In making paper and other pulp products, cellulosic fiber, such as for example wood, is chemically digested in a continuous or a batch process. Usually, the fiber is charged together with a cooking liquid such as a white liquor having certain desirable chemicals for dissolving a majority of the lignin contents of the wood. Pulp so formed is typically washed or rinsed, and separated from the cooking liquid. The filtrate from the rinse forms a weak black liquor.
As used herein, “black liquor” means the waste product that results from separating the pulp from the cooking liquid subsequent to digesting the cellulosic fiber. Black liquor is usually rich in valuable chemicals, some of which can be recovered to produce additional and/or cooking liquid for use in the digester. For example, black liquor can be concentrated by evaporating a major portion of its water contents in an evaporation plant and some of the chemicals can be recovered in the form of Na2CO3. The concentrated black liquor is combusted in a recovering furnace to produce desirable process steam and a smelt having certain desirable chemicals that can be dissolved in water to form a green liquor.
The Na2CO3 (sodium carbonate) is used to produce NaOH (sodium hydroxide), an ingredient used to produce the cooking liquid, by treating the concentrated green liquor with burnt lime, also known as quick lime, (CaO). The causticizing reaction just described and used to produce the sodium hydroxide is shown in Equations 1 and 2.CaO+H2O→+Ca(OH)2+Heat  Equation 1Na2CO3+Ca(OH)2→2NaOH+CaCO3  Equation 2
An additional recovery process is usually applied to “close” the cycle and recover quick lime from the lime sludge (also known as lime mud), which includes CaCO3. NaOH and solutions with dissolved NaOH, such as aqueous NaOH, can also be recovered from the lime sludge. After rinsing, the lime sludge is heated in a lime kiln to evaporate any remaining water, and then heated further in a reburning process to recover the quick lime according to the stoichiometric reaction shown in Equation 3.CaCO3+energy→CaO+CO2  Equation 3
Many kilns used to recover quick lime from lime sludge are heated by a continuous heat source, such as by continuous combustion of natural gas. Fuel costs for kilns heated only by combustion of natural gas are high, and combustion of natural gas only usually leads to peak flame temperatures in excess of 2800° F., which undesirably forms oxides of nitrogen (NON). In addition, some chemicals in the cooking liquid and green liquor (and thus the lime sludge), as well as natural gas, contain sulfur. Consequently, the high combustion temperature of natural gas usually forms oxides of sulfur (SOx) in addition to the NON, which makes compliance with emissions requirements difficult.